One of the important tasks of diagnostic radiation imaging is the ability to differentiate between different materials. Such a task is traditionally applied to absorption images obtained by conventional medical x-ray images devices, which are based on the attenuation through photoelectric absorption of the x-rays penetrating the object to be imaged. However, in the clinically acceptable x-ray imaging energy range (e.g. 10-140 keV), soft tissues (e.g., vessels, cartilages, lungs, breast tissues) have absorption values of similar magnitude. That provides a poor contrast between such materials, which significantly complicates or makes almost unfeasible the material differentiation task.
The problem of low contrast in soft tissues can be addressed with phase contrast imaging (PCI) techniques. The principle of PCI is based on the wave nature of x-rays, where refraction and diffraction properties need to be considered. As an electromagnetic wave, the x-ray is usually characterized by its frequency, amplitude, and phase. When an electromagnetic wave penetrates a medium, its amplitude is attenuated and its phase is shifted. In x-ray technology, the refractive index n of a material can be expressed by a complex numbern=1−δ+iβ.   (1)
The imaginary part β contributes to the attenuation of the amplitude and the real part δ is responsible for the phase shift. It has been shown that β is about 103 to 104 times larger than δ. While conventional medical imaging records only information of β, the information of δ is completely lost.
In recent years, several PCI techniques have been explored to make use of the phase shift through the object. Phase shift, as an addition to absorption information, can be beneficial to material decomposition techniques. Related art material decomposition methods rely on the differences in absorption values of the materials. When the absorption values are so close that ambiguity in material differentiation is created, the phase shift information can be used instead. Additionally spectral techniques (e.g. dual energy techniques), where plurality of images (e.g. absorption and/or phase shift) is created, can be employed to aid the material differentiation. Although, the combination of image plurality and PCI technique can yield significantly better material identification, material misidentification is still possible when code values of different materials are close enough to create the ambiguity. In such a situation, there is a need for improved material decomposition techniques.